JP7724315B2 - Calcium carbonate manufacturing method and calcium carbonate - Google Patents

Description

The present invention relates to a method for producing calcium carbonate and calcium carbonate.

Seawater is rich in minerals, for example, it contains approximately 1,300 ppm of magnesium and approximately 400 ppm of calcium. To make effective use of these minerals, a known method is to produce magnesium hydroxide by reacting seawater with dolomite or other minerals, and then separate and recover the magnesium.

After magnesium has been separated using the above-mentioned methods, seawater (hereinafter referred to as "used water") still contains a large amount of calcium, and efforts are underway to develop technologies to effectively utilize this remaining calcium. Specifically, a technology has been proposed that utilizes the calcium contained in the used water to capture carbon dioxide, thereby enabling industrial applications of the calcium component and contributing to the reduction of carbon dioxide emissions, which is considered an effective measure against recent issues such as global warming (Patent Document 1).

Patent No. 7138256

In order to expand the applications and improve the functionality of calcium carbonate, which is obtained by incorporating (fixing) carbon dioxide into calcium components, calcium carbonate with small particle size, little variation in primary particle size, and good dispersibility is required.

The present invention aims to provide a method for producing calcium carbonate that can achieve small particle size, uniform primary particle size, and high dispersibility in the calcium carbonate obtained through the fixation of carbon dioxide, and also to provide calcium carbonate.

After extensive research, the inventors discovered that the above-mentioned problems could be solved by the following configuration, leading to the completion of the present invention.

In one embodiment, the present invention relates to a method for producing calcium carbonate, which includes a step of contacting seawater, calcium-containing used water remaining after producing magnesium hydroxide from seawater, or a mixture thereof (hereinafter, seawater, used water, and mixtures thereof will also be collectively referred to as "seawater, etc."), with an alkaline agent, and a gas containing carbon dioxide at a pH of 9 or higher and lower than 11.

This calcium carbonate production method makes it possible to produce calcium carbonate with small particle size, consistent primary particle size, and good dispersibility while immobilizing carbon dioxide. While the reason for this is unclear, it is presumed to be as follows: In the contacting step, carbonate ions are generated by contact between the alkaline agent and carbon dioxide, and calcium carbonate particles are produced by contacting these with calcium (calcium ions) in seawater or the like. It is presumed that the relatively mild alkaline conditions of pH 9 or higher but lower than 11 slow down the particle growth rate, resulting in calcium carbonate with small particle size and a uniform particle shape.

In one embodiment, it is preferable to bring the seawater, utilized water, or a mixture thereof, the alkaline agent, and the gas into contact with each other in the following order 1) or 2).
1) After the seawater, utilized water or a mixture thereof is brought into contact with the alkaline agent, the gas is further brought into contact with the seawater, utilized water or a mixture thereof.
2) After the alkaline agent and the gas are brought into contact with each other, the seawater, utilized water, or a mixture thereof is further brought into contact with each other.

By contacting the three components of seawater, an alkaline agent, and a gas containing carbon dioxide in either of the specific sequences 1) or 2), the production of carbonate ions and the growth of calcium carbonate particles can be promoted.

In one embodiment, the alkaline agent is preferably magnesium hydroxide or magnesium oxide. This allows for more efficient particle size reduction of calcium carbonate.

In one embodiment, the temperature in the contact step is preferably 50°C or lower. This allows calcium carbonate to be produced as calcite, providing a range of uses that are suited to the application and function of calcium carbonate.

In one embodiment, the calcium concentration in the seawater, utilized water, or a mixture thereof is preferably 300 ppm or more and 3000 ppm or less, from the viewpoint of calcium carbonate production efficiency.

In one embodiment, the gas may be exhaust gas from a combustion engine.

In one embodiment, the concentration of carbon dioxide in the gas is preferably 1% by volume or more and 20% by volume or less, from the standpoint of calcium carbonate production efficiency.

In one embodiment, the calcium carbonate may be calcite.

In one embodiment, the average particle size of the calcium carbonate measured by laser diffraction is preferably 5 μm or less. This can improve the physical properties of products containing calcium carbonate, allowing for a wider range of applications and higher functionality.

In one embodiment, the present invention provides
The present invention relates to calcium carbonate having a coefficient of variation of primary particle size, represented by the following formula, of 5% to 30%, an average particle size measured by a laser diffraction method of 5 μm or less, and primary particles having a spherical polyhedral or plate-like shape.
Coefficient of variation=(σ _pri /d _pri )×100
(In the formula, σ _pri is the standard deviation (μm) of the primary particle diameter in the electron microscope image, and d _pri is the average value (μm) of the primary particle diameter in the electron microscope image.)

In one embodiment, the present invention provides
The present invention relates to calcium carbonate having a monodispersity of 50% or more and 100% or less, an average particle size of 5 μm or less as measured by a laser diffraction method, and primary particles having a spherical polyhedral or plate-like shape.
Monodispersity=(d _pri /d _ave )×100
(In the formula, d _pri is the average primary particle diameter (μm) measured by an electron microscope image, and d _ave is the average particle diameter (μm) measured by a laser diffraction method.)

The properties of this calcium carbonate allow it to be stabilized, activated, and highly functional, expanding the range of applications for products that use this calcium carbonate. This calcium carbonate is suitable for use in inorganic molded bodies.

The present invention provides a method for producing calcium carbonate that can efficiently immobilize carbon dioxide and has small particle size, little variation in primary particle size, and good dispersibility, as well as calcium carbonate.

1 is a SEM photograph of calcium carbonate of Example 1 of the present invention. 1 is a SEM photograph of calcium carbonate in Example 2 of the present invention. 1 is a SEM photograph of calcium carbonate of Example 3 of the present invention. 1 is a SEM photograph of calcium carbonate in Example 4 of the present invention. 1 is a SEM photograph of calcium carbonate of Example 5 of the present invention. 1 is a SEM photograph of calcium carbonate in Comparative Example 1 of the present invention. 1 is a SEM photograph of calcium carbonate of Comparative Example 2 of the present invention. FIG. 2 is a partial perspective view schematically showing a heating tester.

A method for producing calcium carbonate and calcium carbonate according to one embodiment of the present invention are described below. The present invention is not limited to these embodiments.

<<Method for producing calcium carbonate>>
The method for producing calcium carbonate according to this embodiment includes a step of contacting seawater, calcium-containing water remaining after producing magnesium hydroxide from seawater, or a mixture thereof with an alkaline agent and a gas containing carbon dioxide at a pH of 9 or higher and lower than 11. The production method may include other steps.

<Contact process>
The order of contacting the three components in the contacting step is not particularly limited as long as calcium carbonate is obtained. However, it is preferable to contact the seawater, utilized water, or a mixture thereof with the alkaline agent and the gas in either of the following orders 1) or 2).
1) After the seawater, utilized water or a mixture thereof is brought into contact with the alkaline agent, the gas is further brought into contact with the seawater, utilized water or a mixture thereof.
2) After the alkaline agent and the gas are brought into contact with each other, the seawater, utilized water, or a mixture thereof is further brought into contact with each other.

Hereinafter, the contacting method in step 1) will be described as the direct method, and the contacting method in step 2) will be described as the indirect method.

(direct method)
In the direct method, calcium carbonate is directly produced by contacting seawater or the like with an alkaline agent and then contacting it with a gas. The reaction formula here is as shown in formula (1) below.
Ca ²⁺ +2OH ⁻ +CO ₂ →CaCO ₃ +H ₂ O (1)

Seawater can be directly drawn from the sea near, for example, a magnesium hydroxide manufacturing plant, or it can be filtered or processed before use. Drawing seawater does not have to be from nearby waters; it can be done anywhere seawater is available.

Reused water is a component discharged after producing magnesium hydroxide (Mg(OH) ₂ ) from seawater. Reused water is, for example, discharged from a magnesium hydroxide production plant. When producing magnesium hydroxide, seawater is reacted with calcined dolomite hydrate or hydrated lime. The post-reaction reusable water contains a greater amount of calcium than seawater. For this reason, it is preferable to use reusable water in this production method. The produced magnesium hydroxide is used, for example, as a neutralizing agent for flue gas desulfurization.

The lower limit of the calcium concentration in seawater, etc. is preferably 300 ppm, more preferably 350 ppm, and even more preferably 380 ppm. The upper limit of the calcium concentration is preferably 3000 ppm, more preferably 2800 ppm, and even more preferably 2600 ppm. The lower limit of the calcium concentration is derived primarily from seawater, and the upper limit of the calcium concentration is derived primarily from the utilized water. As mentioned above, the lower limit of the calcium concentration in seawater alone is preferably 300 ppm, more preferably 350 ppm, and even more preferably 380 ppm. The upper limit of the calcium concentration in seawater alone is preferably 500 ppm, more preferably 480 ppm, and even more preferably 450 ppm. The lower limit of the calcium concentration in utilized water alone is preferably 1500 ppm, more preferably 1800 ppm, and even more preferably 2000 ppm. As mentioned above, the upper limit of the calcium concentration in the water to be used alone is preferably 3000 ppm, more preferably 2800 ppm, and even more preferably 2600 ppm. However, this is not limiting, and the calcium concentration can be adjusted by adding a calcium source (e.g., calcium chloride, calcium sulfate, etc.) to the seawater, or by concentrating or diluting the seawater, etc.

The alkaline agent is not particularly limited as long as it can adjust the pH in the contact step to within a predetermined range, and examples thereof include alkaline earth metal or alkali metal hydroxides such as magnesium hydroxide (Mg(OH) ₂ ), calcium hydroxide (Ca(OH) ₂ ), barium hydroxide (Ba(OH) ₂ ), sodium hydroxide (NaOH), and potassium hydroxide (KOH); alkaline earth metal or alkali metal oxides such as magnesium oxide (MgO), calcium oxide (CaO), barium oxide (BaO), sodium oxide (Na ₂ O), and potassium hydroxide (K ₂ O); carbonates such as neutral magnesium carbonate (MgCO ₃ .3H ₂ O), sodium carbonate (NaCO ₃ ), and potassium carbonate (NaCO ₃ ); and industrial waste materials such as lime residue, cement sludge water, washing wastewater from raw cement plants, residual cement, and waste cement. Among these, magnesium hydroxide, magnesium oxide, and neutral magnesium carbonate are preferred as types of alkaline agents that can easily adjust the pH to within a predetermined range, with magnesium hydroxide and magnesium oxide being more preferred, and magnesium hydroxide being even more preferred.

The alkaline agent may be a commercially available synthetic product, a commercially available natural mineral product, or industrial waste. Commercially available natural mineral products and industrial waste are preferred because they emit extremely little carbon dioxide from the time of raw material mining to the completion of production. An example of a natural mineral product as the alkaline agent is finely ground brucite ore (Mg(OH) ₂ ) (10 μm or less). The industrial waste as the alkaline agent is preferably waste magnesium hydroxide or waste magnesium oxide, which is inevitably generated during the manufacturing process of magnesium hydroxide or magnesium oxide, the process of changing brands, or the like. Examples of waste magnesium hydroxide include leaked filter cloth during the filtration process, leaked from the gland packing of a slurry transport pump, and washed products during brand change. Examples of waste magnesium oxide include rotary kiln firing dust (collected dust) and washed products during brand change. Although these industrial wastes have conventionally been discarded as waste, they have a BET specific surface area of 10 to 200 ^{m 2} /g, an average particle size of 10 μm or less, and relatively high reactivity, so they can be effectively used as the alkaline agent in the production method. The alkaline agent may be subjected to treatment such as pulverization or crushing in terms of reactivity and handleability. This treatment may be either wet or dry.

In the direct method, seawater or the like is first brought into contact with an alkaline agent. This contact can be carried out by directly adding the alkaline agent to the seawater or the like, or by first dispersing the alkaline agent in a solvent such as water to form a slurry, which is then added to the seawater or the like. The addition can be carried out all at once or in multiple stages. If the alkaline agent is an oxide, from the standpoint of reactivity, it is preferable to dissolve or disperse it in water or the like to hydrate it and form a slurry before adding it. The concentration of the alkaline agent in the slurry is not particularly limited, and can be in the range of 10 g/L or more and 500 g/L or less.

The amount of alkaline agent added is not particularly limited, as long as it is set so that the amount of hydroxide ions necessary for reaction with the amount of calcium ions in seawater or the like is obtained according to the above reaction formula (1) and the pH of the contact step is within a predetermined range. For example, when the alkaline agent is magnesium hydroxide (Mg(OH) ₂ ), the molar ratio of Mg in the alkaline agent to Ca ²⁺ in seawater or the like (Mg/Ca ²⁺ ) is preferably 1.0 or less, more preferably 0.9 to 1.0, and even more preferably 0.95 to 1.0. It is particularly preferable that the molar ratio is 1.0, i.e., an equimolar amount. By setting the molar ratio within the above range, coprecipitation of the reaction product of Mg in the alkaline agent can be suppressed, allowing high-purity calcium carbonate to be produced. The amount of magnesium hydroxide added per 10 L of water to be used with a ^{Ca 2+} concentration of 2500 ppm is preferably 32.7 g to 36.4 g, more preferably 34.5 g to 36.4 g. Furthermore, when the alkaline agent is magnesium oxide (MgO), the molar ratio (Mg/Ca ²⁺ ) of Mg in the alkaline agent to Ca ²⁺ in the seawater or the like can suitably be set in the same range as for magnesium hydroxide. The amount of magnesium oxide to be added per 10 L of seawater with a ^{Ca 2+} concentration of 400 ppm is preferably 3.6 g to 4.0 g, and more preferably 3.8 g to 4.0 g. Even when the alkaline agent is other than magnesium hydroxide or magnesium oxide, the amount to be added can be set based on the same considerations.

In this production method, when seawater or the like is brought into contact with an alkaline agent, the pH becomes equal to or greater than 9 and less than 11. The lower limit of the pH is preferably 9.2, more preferably 9.4, even more preferably 9.6, and particularly preferably 9.8. The upper limit of the pH is preferably 10.9, more preferably 10.8, even more preferably 10.6, even more preferably 10.4, and particularly preferably 10.2.

The temperature at which seawater or the like is brought into contact with the alkaline agent is not particularly limited; it may be at the temperature of the seawater as drawn, left to stand for a while at room temperature, or heated. The temperature is preferably 50°C or lower, and more preferably in the range of 0°C to 30°C. By keeping the temperature at 50°C or lower, calcium carbonate can be produced as calcite with small particle size and good dispersibility. From the standpoints of reactivity and efficiency, a temperature around room temperature is preferred, with a temperature of 10°C to 30°C being more preferred, and a temperature of 15°C to 28°C being even more preferred.

The gas to be contacted in the direct method is not particularly limited as long as it contains carbon dioxide. Suitable gases include exhaust gas from combustion engines such as boilers and thermal power plants (power plants that use heat from the combustion of fossil fuels such as petroleum, coal, and liquefied natural gas (LNG) or biomass), blast furnaces, and dryers. Exhaust gas from combustion engines is preferred because it has a high carbon dioxide concentration and provides good calcium carbonate production efficiency in the contact step. The lower limit of the carbon dioxide concentration in the gas is preferably 1% by volume, more preferably 2% by volume, more preferably 5% by volume, and particularly preferably 8% by volume. While a higher upper limit is preferable, it may be 30% by volume, 20% by volume, 15% by volume, or 12% by volume. This production method not only reduces emissions by immobilizing carbon dioxide, but also promotes industrial use in the production of calcium carbonate.

In the direct method, gas contact can be carried out by blowing (bubbling) the gas into seawater or the like that has been in contact with an alkaline agent. The gas flow rate and blowing time can be appropriately set taking into account the carbon dioxide concentration, reaction efficiency, and the progress of the carbonation reaction of reaction formula (1). For example, for 10 L of seawater or the like, the gas flow rate is preferably 0.5 L/min to 30.0 L/min, more preferably 2.0 L/min to 25.0 L/min, and even more preferably 3.0 L/min to 20.0 L/min. It is sufficient to introduce _CO2 in an amount greater than the theoretical number of moles required until the Ca2 ⁺ in the seawater is completely precipitated as calcium carbonate, and the minimum gas blowing time is determined by the Ca2 ⁺ concentration and amount used in the seawater or the like and the _CO2 concentration in the exhaust gas. For example, when _CO2 having a concentration of 10% by volume in the exhaust gas is injected at 5.0 L/min into 10 L of water containing 2500 ppm Ca2 ⁺ , an injection time of 0.5 hours or more is required, preferably 1.0 to 8.0 hours, and more preferably 2.0 to 6.0 hours.

By going through the above contacting process, calcium carbonate with small particle size, little variation in primary particle size, and good dispersibility can be produced. The resulting calcium carbonate may be filtered, washed, dried, crushed, classified, etc.

(indirect method)
In the indirect method, an alkaline agent is brought into contact with a gas to first generate a carbonate salt (carbonate ion) of the alkaline agent, and then the resulting product is brought into contact with seawater or the like to indirectly generate calcium carbonate. The reaction formulas used here are shown in the following formulas (2-1) and (2-2).
2OH ^- +CO ₂ →CO ₃ ^2- +H ₂ O (2-1)
Ca ²⁺ +CO ₃ ^2- →CaCO ₃ (2-2)

In the indirect method, an alkaline agent is brought into contact with a gas (carbon dioxide) to produce carbonate (carbonate ions), which is then brought into contact with calcium (calcium ions) from seawater or other sources to carry out salt exchange, producing calcium carbonate. Below, we will explain how the indirect method differs from the direct method.

The contact between the alkaline agent and the gas (reaction formula (2-1)) can be achieved by dissolving or dispersing the alkaline agent in a solvent such as water to form a slurry or solution (hereinafter collectively referred to as "slurry, etc."), and then blowing (bubbling) the gas into the slurry, etc. The concentration of the alkaline agent in the slurry, etc. is not particularly limited and can be set appropriately in the range of 10 g/L or more and 100 g/L or less.

The gas flow rate and injection time can be set appropriately taking into account the carbon dioxide concentration, reaction efficiency, and the progress of the carbonate (carbonate ion) production reaction of reaction formula (2-1). For example, for 10 L of slurry, the gas flow rate is preferably 1.0 L/min to 30.0 L/min, more preferably 2.0 L/min to 25.0 L/min, and even more preferably 3.0 L/min to 20.0 L/min. The injection time is preferably 0.5 hours to 8.0 hours, more preferably 1.0 hours to 6.0 hours, and even more preferably 2.0 hours to 4.0 hours.

For example, when the alkaline agent is magnesium hydroxide (Mg(OH) ₂ ), a slurry of neutral magnesium carbonate (MgCO ₃ .3H ₂ O) is obtained as a carbonate via the reaction of reaction formula (2-1). The obtained slurry of neutral magnesium carbonate may be subjected to the reaction of reaction formula (2-2) as is, but it is preferable to subject the slurry to a particle size reduction treatment such as wet grinding in order to improve the reaction activity before subjecting the slurry to the reaction of reaction formula (2-2). The wet grinding treatment can be carried out by a conventionally known method such as a ball mill. The particle size reduction treatment may be carried out appropriately depending on the shape, size, etc. of the carbonate obtained via reaction formula (2-1).

Next, the carbonate is contacted with seawater or the like (reaction formula (2-2)). The contact may be carried out by adding a slurry of the carbonate to seawater or the like, or by adding seawater to the slurry. The amount of carbonate added is not particularly limited, as long as it is set so that the amount of carbonate ions necessary for the reaction with the amount of calcium ions in seawater or the like is obtained according to reaction formula (2-2) above, and the pH after addition is within a predetermined range. For example, when the carbonate is neutral magnesium carbonate, the molar ratio of Mg in the carbonate to Ca ²⁺ in seawater or the like (Mg/Ca ²⁺ ) is preferably 1.0 or less, more preferably 0.9 or more and 1.0 or less, and even more preferably 0.95 or more and 1.0 or less. It is particularly preferable that the molar ratio is 1.0, i.e., an equimolar amount. By setting the molar ratio within the above range, coprecipitation of the reaction product of Mg in the carbonate can be suppressed, and high-purity calcium carbonate can be produced. The amount of neutral magnesium carbonate to be added to 10 L of utilized water having a Ca ²⁺ concentration of 2500 ppm is preferably 77.6 g or more and 86.2 g or less, and more preferably 81.9 g or more and 86.2 g or less.

When the carbonate comes into contact with seawater or the like, the pH immediately after contact is preferably at least 9 and less than 11. The lower limit of the pH is preferably 9.1, and more preferably 9.2. The upper limit of the pH is preferably 10.0, more preferably 9.8, even more preferably 9.6, and particularly preferably 9.4.

The salt exchange reaction caused by contact between carbonate and seawater, etc., proceeds relatively quickly. The reaction time should be set long enough to allow the salt exchange reaction to proceed sufficiently, and can be set to 0.1 hours or more, preferably 0.5 hours to 8.0 hours, and more preferably 1.0 hours to 6.0 hours.

Calcium carbonate
Each aspect of calcium carbonate will be described below. Calcium carbonate having the following characteristics can be efficiently produced by the above-described method for producing calcium carbonate. The methods for measuring or evaluating the standard deviation of primary particle diameters in electron microscope images, the average value of primary particle diameters in electron microscope images, and the average particle diameter and shape of primary particles by laser diffraction method shown below are as described in the Examples.

(First Aspect)
The calcium carbonate has a coefficient of variation of primary particle size, represented by the following formula, of 5% or more and 30% or less, an average particle size measured by a laser diffraction method of 5 μm or less, and the primary particles have a spherical polyhedral or plate-like shape.
Coefficient of variation=(σ _pri /d _pri )×100
(In the formula, σ _pri is the standard deviation (μm) of the primary particle diameter in the electron microscope image, and d _pri is the average value (μm) of the primary particle diameter in the electron microscope image.)

The lower limit of the coefficient of variation is preferably 6%, more preferably 7%, and even more preferably 8%. The upper limit of the coefficient of variation is preferably 28%, more preferably 26%, and even more preferably 25%.

The lower limit of the standard deviation of the primary particle diameter in the electron microscope image is preferably 0.10 μm, more preferably 0.12 μm, and even more preferably 0.14 μm. The upper limit of the standard deviation of the primary particle diameter is preferably 0.95 μm, more preferably 0.90 μm, and even more preferably 0.85 μm.

The lower limit of the average primary particle diameter measured by the electron microscope image is preferably 0.1 μm, more preferably 0.5 μm, and even more preferably 0.8 μm. The upper limit of the average primary particle diameter is preferably 5.0 μm, more preferably 4.5 μm, and even more preferably 4.0 μm.

The lower limit of the average particle size measured by the laser diffraction method is preferably 0.5 μm, more preferably 1.0 μm, and even more preferably 1.5 μm. The upper limit of the average particle size measured by the laser diffraction method is preferably 4.8 μm, more preferably 4.6 μm, and even more preferably 4.0 μm.

The primary particles are spherical polyhedrons or plates. A spherical polyhedron is a particle that is spherical overall, but whose surface is not spherical, but is formed by a combination of polygonal planes.

(Second Aspect)
The calcium carbonate has a monodispersity of 50% or more and 100% or less, as determined by the following formula, an average particle size of 5 μm or less as measured by a laser diffraction method, and primary particles having a spherical polyhedral or plate-like shape.
Monodispersity=(d _pri /d _ave )×100
(In the formula, d _pri is the average primary particle diameter (μm) measured by an electron microscope image, and d _ave is the average particle diameter (μm) measured by a laser diffraction method.)

The lower limit of the monodispersity is preferably 55%, more preferably 60%. The upper limit of the monodispersity is preferably 100%, more preferably 98%.

For the second aspect, the average primary particle size measured by electron microscope images, the average particle size measured by laser diffraction, and the shape of the primary particles can suitably be the same as those for the first aspect.

(Common matters between the first and second aspects)
Examples of the crystal structure of calcium carbonate include calcite and aragonite, with calcite being preferred.

The BET specific surface area of calcium carbonate is preferably 2 m ² /g or more and 20 m ² /g or less, more preferably 3 m ² /g or more and 18 m ² /g or less, and even more preferably 4 m ² /g or more and 15 m ² /g or less.

The P funnel flow time for the calcium carbonate is preferably 7 to 10 seconds, more preferably 7.5 to 9.8 seconds, and even more preferably 8 to 9.5 seconds. Because this calcium carbonate has excellent dispersibility, it exhibits excellent fluidity, particularly in concrete applications.

The uses of calcium carbonate are not particularly limited, but include fillers for resins, compounding agents for inorganic molded bodies (aggregates for concrete structures (cement) and compounding agents for building materials), thickeners for sealants, etc. Adding calcium carbonate to these products can expand their applications and improve their functionality.

Although not particularly limited, it is preferable to use a thermoplastic resin as the resin. Examples of the thermoplastic resin include, but are not limited to, acrylic resins such as homopolymers or copolymers of (meth)acrylic acid esters, such as polymethyl methacrylate (PMMA), ethylene/(meth)acrylic acid ester copolymers, and methyl acrylate/methyl methacrylate copolymers; high-density polyethylene, low-density polyethylene, linear low-density polyethylene, very-low-density polyethylene, EVA (ethylene vinyl acetate resin), EEA (ethylene ethyl acrylate resin), EMA (ethylene methyl acrylate copolymer resin), EAA (ethylene acrylic acid copolymer resin), and ultra-high molecular weight polyethylene. Examples of suitable resins include polyethylene resins (PE) such as polyethylene homopolymer and ethylene-propylene copolymer; polypropylene resins (PP) such as polypropylene homopolymer and ethylene-propylene copolymer; ABS resin (acrylonitrile-butadiene-styrene copolymer); polystyrene resin; polycarbonate resin; polyphenylene resin; polyester resins such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene naphthalate (PEN); and polyamide resins including various nylons such as nylon 6, nylon 66, nylon 610, nylon 612, nylon 11, nylon 12, and nylon 46. These resins may be used alone or in combination of two or more.

(Aggregate for concrete structures (cement))
The concrete structure is composed of a hardened hydraulic composition. The hydraulic composition is made of a powder containing calcium carbonate and at least one of blast furnace slag, expansive agent, slaked lime, quicklime, fly ash, and Portland cement. The calcium carbonate described above can be suitably used as the calcium carbonate.

In addition to the above hydraulic composition, aggregates such as sand and gravel, chemical admixtures for concrete, and fiber materials made from metals or polymeric materials may also be added to form a hydraulic composition mixture.

A hardened hydraulic composition is obtained by hardening a paste obtained by kneading the hydraulic composition with water. A hardened hydraulic composition mixture is obtained by hardening a mixture (equivalent to fresh mortar or fresh concrete) obtained by kneading the hydraulic composition mixture with water, and is equivalent to mortar or concrete.

The proportion of calcium carbonate in the powder (the ratio of calcium carbonate to cement) is within the range of 1% to 60% by mass, preferably 3% to 50% by mass, and more preferably 5% to 40% by mass.

It is desirable to use ground granulated blast furnace slag used in JIS (Japanese Industrial Standards) R5211 "Blast furnace cement" or ground granulated blast furnace slag conforming to JIS A6206 "Blast furnace slag for concrete." It is also desirable to use blast furnace slag with a specific surface area of 2000 to 10000 ^{cm 2} /g, preferably 3500 to 7000 cm ² /g.

For example, the expansive additive specified in JIS A6202 "Expansive additives for concrete" can be used. It is desirable to add the expansive additive in an amount of 2 to 9% by mass based on the total hydraulic composition.

For example, slaked lime specified in JIS R9001 "Industrial Lime" may be used. Furthermore, quicklime becomes slaked lime when it comes into contact with water, so quicklime specified in JIS R9001 "Industrial Lime" may be used instead of slaked lime. In this case, it is advisable to correct the amount of water required to convert quicklime into slaked lime.
The fly ash used may be one that conforms to JIS A6201 "fly ash for concrete," for example.

Ordinary Portland cement is used as Portland cement, but other types of Portland cement, such as high-early-strength Portland cement, ultra-high-early-strength Portland cement, moderate-heat Portland cement, low-heat Portland cement, and sulfate-resistant Portland cement, as specified in JIS R5210 "Portland cement," and JIS R5214 "Ecocement" can also be used.

If the hydraulic composition contains Portland cement, the proportion of Portland cement in the powder other than calcium carbonate should be 70% by mass or less, and preferably 30% by mass or less.

Furthermore, when using Portland cement and blast furnace slag or fly ash, the components may be pre-mixed, such as JIS R5211 "Blast Furnace Cement" or JIS R5213 "Fly Ash Cement," and these may be used alone or in combination.

By using calcium carbonate with the above characteristics, the hydraulic composition and hydraulic composition mixture exhibit good fluidity, and the resulting hardened concrete can exhibit excellent compressive strength.

The density of the concrete structure is preferably 0.7 g/cm ³ or more and 2.0 g/cm ³ or less, more preferably 0.8 g/cm ³ or more and 1.8 g/cm ³ or less, and even more preferably 0.9 g/cm ³ or more and 1.6 g/cm ³ or less.

(Building material compounding agent)
The building material is preferably a molded board for building materials. The molded board preferably contains a hydraulic material, a siliceous material, a reinforcing fiber material, and calcium carbonate.

(hydraulic material)
Examples of hydraulic materials include cementitious materials, gypsum, lime, slag, etc. Examples of cementitious materials include commonly used cements, such as ordinary Portland cement, high-early-strength cement, moderate-heat cement, fly ash cement, blast furnace slag cement, and alumina cement. Examples of gypsum include anhydrous gypsum, hemihydrate gypsum, and dihydrate gypsum. Examples of slag include blast furnace slag and converter slag. These hydraulic materials can be used alone or in combination of two or more.

The content of hydraulic material is preferably 5% by mass or more and 45% by mass or less, more preferably 8% by mass or more and 42% by mass or less, and even more preferably 10% by mass or more and 40% by mass or less, based on the total amount of materials constituting the molded board. By keeping the content of hydraulic material within this range, it is possible to improve the physical properties of the molded board, such as bending strength and peel strength, and to prevent the molded board from becoming too high in bulk density, thereby improving workability during construction.

(siliceous material)
Examples of siliceous materials include materials containing a large amount of SiO2, such as silica sand, silica powder, silica fume, fly ash, diatomaceous earth, layered silicates (e.g., mica, talc, kaolin, bentonite), perlite, wollastonite, and lightweight aggregates (e.g., fly ash balloons, perlite, shirasu balloons, glass foam, etc.). These _siliceous materials can be used alone or in combination of two or more. Talc, mica, and wollastonite can also be used as reinforcing fiber materials, which will be described later.

The content of the siliceous material is preferably 10% by mass or more and 55% by mass or less, more preferably 12% by mass or more and 50% by mass or less, and even more preferably 15% by mass or more and 45% by mass or less, based on the total amount of materials constituting the molded board. If the content of the siliceous material is within the above range, it is possible to set the bending strength, bulk density, water absorption, dimensional stability, etc. of the molded board within the desired range. Note that when lightweight aggregates with a unit volume mass of 0.5 g/cm3 or less, such as perlite, fly ash balloons, and shirasu balloons, are blended as the siliceous material, it is preferable to use other siliceous materials in combination so that the content of the lightweight aggregate is 20% by mass or ^less , based on the total amount of materials constituting the molded board, in order to prevent the bulk density from becoming too low and weakening strength such as bending strength and peel strength.

(Reinforcing fiber material)
Examples of reinforcing fiber materials that can be used include pulps such as softwood pulp, hardwood pulp, fibrillated pulps thereof, and pulp obtained by defibrating waste paper, organic reinforcing fiber materials such as vinylon fiber, acrylonitrile fiber, and polypropylene fiber, and inorganic reinforcing fiber materials such as rock wool and glass fiber. These reinforcing fiber materials can be used alone or in combination of two or more.

To improve the strength and toughness of the molded board, the content of the reinforcing fiber material is preferably 2% by mass or more and 30% by mass or less, more preferably 3% by mass or more and 26% by mass or less, and even more preferably 4% by mass or more and 22% by mass or less, based on the total amount of materials constituting the molded board. By keeping the content of the reinforcing fiber material within this range, it is possible to achieve sufficient reinforcing effect while suppressing the protrusion of fibers from the surface of the molded board, thereby improving smoothness. When inorganic reinforcing fiber material with an average length of 1 mm to 50 mm is blended as the reinforcing fiber material, it is preferable to use other reinforcing fiber materials in combination so that the content is 10% by mass or less, based on the total amount of materials constituting the molded board, in order to improve the smoothness of the molded board.

(Calcium carbonate)
As the calcium carbonate, the above-mentioned calcium carbonate can be suitably used.

The calcium carbonate content is preferably 5% to 60% by mass, more preferably 8% to 55% by mass, and even more preferably 12% to 50% by mass, based on the total amount of materials constituting the molded board. By incorporating low-thermal-conductivity calcium carbonate within this range, the strength and fire resistance of the molded board can be improved.

(optional ingredient)
In addition to the above materials, various materials can be blended into the molded board depending on the purpose to impart various functions, such as hollow resin bodies, wood chips, wood flour, resin powder, antifoaming agents, flocculants, water repellents, thickeners (methyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose, etc.), dispersants, etc. It is also possible to add recycled materials made by crushing scraps generated during the processing of the molded board as appropriate.

The bulk density of the molded plate is preferably 0.7 g/cm ³ or more and 2.0 g/cm ³ or less, more preferably 0.8 g/cm ³ or more and 1.8 g/cm ³ or less, and even more preferably 0.9 g/cm ³ or more and 1.6 g/cm ³ or less.

(Method for manufacturing molded plate)
The method for producing the molded plate according to this embodiment is not particularly limited, and commonly used methods such as papermaking, extrusion molding, flow-on molding, casting, and press (compression) molding can be used. The molded plate can be obtained by subjecting a green sheet molded by these methods to press dehydration or patterning by embossing, etc., followed by curing at room temperature, steam curing, autoclave curing, etc. The molded plate can then be dried and, if necessary, shaped or painted.

(Uses of molded plates)
The uses of the molded board are not particularly limited, and it can be suitably used as a performance maintaining material for wall construction, flooring, roofing, various boards, exterior decorative members, interior and exterior finishing materials such as fittings, sealing materials, heat insulating materials, sound absorbing materials, waterproofing materials, etc. The molded board is preferably a cementitious molded board containing a cementitious material, and more preferably a calcium silicate molded body.

The present invention will be described in detail below using examples, but the present invention is not limited to the following examples as long as it does not deviate from the gist of the invention. Measurements and evaluations of physical properties, etc. were carried out as follows.

[Analysis of seawater and used water]
Table 1 shows the results of analyzing the components and pH of seawater collected from the Seto Inland Sea near Konoshima Chemical Co., Ltd. and the used water discharged from the company's magnesium hydroxide manufacturing process. The used water here refers to the supernatant remaining after reacting Mg ²⁺ , which is present in seawater at approximately 1,300 ppm, with an alkaline agent (such as calcium hydroxide) to precipitate and recover magnesium hydroxide "Mg(OH) ₂ ." The components of the seawater and used water were analyzed by measuring Ca ²⁺ using ICP-AES (Hitachi High-Tech Science Corporation, inductively coupled plasma atomic emission spectrometer, SPECTROBLUE FMS36). The pH of the seawater and used water was also measured using a pH meter (Toa DKK Corporation, main unit: multi-water quality meter MM-43X, pH electrode: GST5841C).

[Evaluation of raw materials, calcium carbonate (filler), and resin composition]
The calcium carbonate and other materials obtained in the Examples and Comparative Examples were subjected to the following analyses. The results of the analyses are shown in Tables 2 and 3 and Figures 1 to 7.

(1) BET Specific Surface Area A sample powder was pretreated in a nitrogen gas atmosphere at about 130°C for about 30 minutes using an 8-tube preheat unit (manufactured by MOUNTECH), and the BET specific surface area (m ² /g) was measured by the nitrogen gas adsorption method using a Macsorb HM Model-1208 (manufactured by MOUNTECH) as a BET specific surface area measuring device.

(2) Average particle size by laser diffraction method 50 mL of ethanol was placed in a 100 mL beaker, about 0.2 g of sample powder was added, and a dispersion was prepared by ultrasonic treatment (UD-201, manufactured by Tomy Seiko Co., Ltd.) for 3 minutes. This prepared solution was measured using a laser diffraction particle size distribution analyzer (Microtrac HRA Model 9320-X100, manufactured by Nikkiso Co., Ltd.) to measure the volume-based _D50 value as the average particle size (μm).

(3) XRD Measurement The sample powder was pressed and fixed onto a predetermined sample stage with a spatula, and then measured using an XRD device (MiniFlex600-C manufactured by Rigaku Corporation) to perform identification analysis as a crystalline substance.

(4) Scanning Electron Microscope Double-sided tape was attached to an aluminum sample stage, and the sample powder was applied to the tape by tracing it with a spatula. After platinum deposition, particle images of the sample powder were photographed at 5,000 magnifications using a scanning electron microscope (FE-SEM: S-4700 manufactured by Hitachi, Ltd.). SEM photographs are shown in Figures 1 to 7.

(5) Measurement of Coefficient of Variation Twenty particles were randomly selected from the SEM photograph obtained in (4) above, and the average value (d _pri ) and standard deviation (σ _pri ) of the primary particle diameter (longest diameter of the primary particles) were determined using image analysis software (Image J). Here, the longest diameter of the primary particles was determined by measuring the dimensions of the particles to be measured from all directions and taking the dimension of the particle in the direction in which the particle size was largest (i.e., the longest diameter). Furthermore, the coefficient of variation of the primary particle diameter was calculated using the following formula.
Coefficient of variation=(σ _pri /d _pri )×100
(In the formula, σ _pri is the standard deviation (μm) of the primary particle diameter in the electron microscope image, and d _pri is the average value (μm) of the primary particle diameter in the electron microscope image.)

(6) Measurement of monodispersity: Twenty particles were randomly selected from the SEM photograph obtained in (4) above, and the average value of the primary particle diameter (the longest diameter of the primary particles) was calculated using image analysis software (Image J). Furthermore, (2) the average particle diameter measured by the laser diffraction method and the monodispersity were calculated using the following formula:
Monodispersity=(d _pri /d _ave )×100
(In the formula, d _pri is the average primary particle diameter (μm) measured by an electron microscope image, and d _ave is the average particle diameter (μm) measured by a laser diffraction method.)

(7) Bending test (bending stress, elastic modulus)
PP (polypropylene) resin (trade name: BC-6D, manufacturer: Japan Polypropylene Corporation) was used as the polyolefin resin. 20 parts by mass of calcium carbonate (filler) powder, described below, was melt-kneaded for 5 minutes at 180 ° C. using a Labo Plastomill (manufactured by Toyo Seiki Seisakusho) with respect to 100 parts by mass of PP resin. The resulting mixture was press-molded at 180 ° C. to produce a 3 mm thick sheet molded body. Using test pieces (length 120 mm, width 13 mm) punched into strips from this sheet molded body, the maximum bending stress and elastic modulus were measured in accordance with the three-point bending test (JIS-K-7171). The target values were a bending stress of 33 N/mm ² or more and an elastic modulus of 1400 N/mm ² or more.

[Example 1]
Six liters of water discharged from Konoshima Chemical Co., Ltd. was placed in an 8-liter SUS container with a baffle and stirred at 350 rpm using a stirrer equipped with one turbine blade at room temperature of 25°C. While stirring, 21.8 g of waste magnesium hydroxide "Mg(OH) ₂ " powder (BET specific surface area: 38 ^m2 /g, average particle size: 3.5 μm) generated within the company was added (Mg/Ca2 ⁺ molar ratio: 1.0). Immediately after addition, a pH meter measurement revealed a pH of 10.1. A flue gas extraction pipe was connected to the exhaust outlet of an LNG-fueled steam production boiler. A laboratory blower was used to draw in the flue gas. Measurements using a _CO2 concentration meter (XP-3140, manufactured by New Cosmos Electric Co., Ltd.) revealed a _CO2 concentration of 10% by volume. The exhaust gas was introduced into the aforementioned 8 L SUS vessel using a laboratory blower at a rate of 3.3 L/min, and the reaction was carried out for 4 hours. Thereafter, the mixture was filtered, washed with water in an amount approximately five times the solid content, and dried at 110°C for 12 hours to obtain a sample powder of the product.

[Example 2]
38 L of seawater collected from the Seto Inland Sea near Konoshima Chemical Co., Ltd. was placed in a 50 L SUS vessel equipped with a baffle, and then a slurry prepared by dispersing 15.0 g of dust powder (BET specific surface area: 75 m ² /g, average particle size: 3.3 μm) of waste magnesium oxide "MgO" generated within the company in 200 mL of pure water was added (Mg/Ca ²⁺ molar ratio: 1.0), and the pH immediately afterwards was 10.0. Except for this, the same operation as in Example 1 was carried out to obtain a sample powder of the product.

[Example 3]
A flue gas extraction pipe was connected to the exhaust outlet of an LPG-fueled hot air dryer, and the exhaust gas was drawn in using a laboratory blower. Measurements were made with a _CO2 concentration detector tube (No. 2H, manufactured by Gastec Corporation), which showed a _CO2 concentration of 2% by volume. A sample powder of the product was obtained by the same procedure as in Example 1, except that the flue gas was introduced into the aforementioned 8-liter SUS container at a rate of 15.3 L/min using the laboratory blower.

[Example 4]
A slurry of 165.3 g of waste magnesium hydroxide powder (Mg(OH) ₂ ) generated in-house was dispersed in 6 L of pure water and placed in an 8 L SUS container with a baffle. Exhaust gas was introduced at a rate of 3.3 L/min using a laboratory blower at room temperature of 25°C, and the mixture was allowed to react for 120 minutes to produce a slurry of neutral magnesium carbonate ( _MgCO3.3H2O ). One liter of this slurry was placed in a 4 L pot mill packed with 1 kg of 8 mm diameter zirconia balls and wet-ground at 90 rpm for 24 hours. A portion of _the wet-ground slurry was filtered and dried to obtain neutral magnesium carbonate with a BET specific surface area of 21 ^m2 /g and an average particle size of 7.8 μm. To the 8 L SUS container, 6 L of water was added and the above-mentioned wet-pulverized neutral magnesium carbonate slurry was added so that the " _MgCO3.3H2O " content was 51.7 _g . The pH value immediately after addition was 9.3. The same procedure as in Example 1 was then carried out, except that the reaction was completed by stirring for 1 hour, to obtain a sample powder of the product.

[Example 5]
250 g of coarsely crushed brucite ore (under 2 mm) and 1 L of pure water were placed in a 4 L pot mill filled with 1 kg of 8 mm diameter zirconia balls and wet-ground for 24 hours at 90 rpm. A portion of the wet-ground slurry was filtered and dried to obtain brucite with a BET specific surface area of 15 m ² /g and an average particle size of 3.8 μm. 6 L of water and the wet-ground brucite slurry were added to the 8 L SUS container so that the "Mg(OH) ₂ " content was 22.0 g. The pH immediately after addition was 10.6. Otherwise, the same procedures as in Example 1 were repeated to obtain a sample powder of the product.

[Comparative Example 1]
Six liters of water discharged from the company was placed in an 8-liter SUS container with a baffle, and 27.7 g of quicklime hydrate slag "Ca(OH) ₂ " generated in the company (slag that passed a 45 μm sieve after hydrating quicklime, BET specific surface area: 14 ^m2 /g, average particle size: 50 μm) was added and mixed (Ca/Ca2 ⁺ molar ratio: 1.0). The pH value immediately after mixing was 12.6. Except for this, the same procedure as in Example 1 was performed to obtain a sample powder of the product.

[Comparative Example 2]
6980 g of slaked lime powder (calculated as CaO), 630 g of aragonite seed crystal powder, and 500 g of disodium hydrogen phosphate dodecahydrate were prepared and each was stirred into a 220 L SUS vessel equipped with a baffle and pre-filled with 180 L of water to prepare a mixed slurry of raw materials. The mixture was then heated to 70 °C and stirred at 150 rpm using a stirrer equipped with one turbine blade at that temperature. A flue gas extraction pipe was connected to the exhaust outlet of an LNG-fueled steam production boiler. While drawing in the exhaust gas using a test blower, the CO2 concentration was measured using a _CO2 concentration meter (XP-3140 manufactured by New Cosmos Electric Co., Ltd.). The _CO2 concentration was 10% by volume. The flue gas was introduced into the aforementioned 220 L SUS vessel using a test blower at a rate of 100 L/min, and the reaction was carried out for 7 hours. Thereafter, the solid was filtered, washed with about 5 times the amount of water relative to the solid content, dried at 110° C. for 24 hours, and pulverized to obtain a powder sample of the product.

As shown in Tables 2 and 3, the calcium carbonate obtained in the Examples had smaller particle sizes than the Comparative Examples, less variation in primary particle size, and better dispersibility. It can also be seen that the physical properties of the resin compositions containing calcium carbonate as a filler were improved compared to the Comparative Examples.

<Evaluation for concrete use>
Using the calcium carbonate of Example 1 (spherical polyhedron) and Comparative Example 2 (acicular), measurements of P funnel flow time, production of cement molded bodies, and compressive strength tests were carried out. The results are shown in Table 5. In Table 5, "-" indicates that evaluation could not be performed. In the tables below, except for Table 5, "-" indicates that the corresponding component was not used.

<P funnel flow time>
(Preparation Example 1) Preparation of Cement Milk 1 2 kg of cement (Tokuyama Corporation, "Normal Portland Cement (N)") was added to 1600 mL of water over approximately 20 seconds, and mixed for 3 minutes from the start of addition using a stirrer (Yamato Scientific Co., Ltd., "Labo Stirrer (LR500B)"). After stopping the stirring and leaving it to stand for 3 minutes, the mixture was manually stirred 10 times using a stirring rod (As One Corporation, "Stirring rod (POM) φ10 × 300 mm") to prepare cement milk 1.

(Preparation Examples 2 to 7) Preparation of Cement Milks 2 to 7 The types and amounts of calcium carbonate shown in Table 4 below were added to 1600 mL of water, and the mixture was stirred manually with the stirring rod for approximately 30 seconds, and then stirred at 400 rpm using the stirrer to obtain a mixture. 2 kg of cement (Tokuyama Corporation, "Ordinary Portland Cement (N)") was added to this mixture over approximately 20 seconds, and mixed with the stirrer for 3 minutes from the start of addition. Stirring was stopped and the mixture was allowed to stand for 3 minutes, after which cement milks 2 to 7 were prepared by manually stirring 10 times with the stirring rod.

<P funnel flow time test method>
The P funnel flow time was measured in accordance with "Test method for fluidity of injection mortar for prepacked concrete (method using P funnel)" (JSCE-F521-1999). The outlet of the P funnel was held with a finger, and each of the prepared cement milks was poured up to the marked line of the P funnel (1750 ml). Measurement was started as soon as the finger was released using a time watch, and the time until the cement milk was discharged from the P funnel was measured.

<Production of cement molded body>
[Example 6-1]
400 mL of the prepared cement milk 1 was poured into a cylindrical polyethylene bag (diameter approximately 50 mm × length approximately 550 mm × thickness approximately 0.05 mm) up to the marked line. After injecting as much air as possible and sealing the bag, the bag was hung in an incubator set at 22°C. The bag was left hanging in the incubator for 28 days to harden the contents, producing a total of three cement molded bodies. The resulting cement molded bodies were cylindrical, with a diameter of approximately 5 cm and a length of approximately 20 cm.

[Examples 6-2 to 6-3 and Comparative Examples 6-1 to 6-4]
A cement molded body was produced in the same manner as in Example 6-1, except that the cement milk shown in Table 5 below was used. In Comparative Example 6-4, the cement milk was not discharged from the P funnel, and therefore evaluation could not be performed.

(density)
The density was measured in accordance with JIS A 5430:2008 (apparent density test).

(Compression strength test)
The compressive strength of the obtained cement molded body was measured in accordance with JIS A 1108:2018 (compression test method for concrete).

In the cement moldings of the examples, even though the calcium carbonate content was increased, there was no significant increase in the P funnel flow time compared to Comparative Example 6-1, which did not contain calcium carbonate, and they had good fluidity. On the other hand, in Comparative Examples 6-2 to 6-4, the fluidity was significantly reduced.

<Evaluation for building materials>
(Manufacturing of molded plates)
Molded plates were manufactured by extrusion molding according to the following procedure. The amounts of the components used are all expressed in "parts by mass" unless otherwise specified. In the table below, "-" indicates that the corresponding component was not used.

[Example 7-1] Production of molded plate by extrusion molding method The materials shown in Table 6 below were charged into an omnimixer, and the raw materials were dry-mixed for 3 minutes. The calcium carbonate from Example 1 was used as calcium carbonate. Next, water was added and wet-mixed for 2 minutes. The wet-mixed raw materials were kneaded in an Ishikawa extruder and then extrusion-molded using the Ishikawa extruder. This produced a molded body (long side 600 mm x short side 190 mm x thickness 13 mm). After obtaining the molded body, it was placed in a thermo-hygrostat set at 60°C/98% and subjected to primary curing, and then autoclave curing was performed for 12 hours at a pressure increased to 9 kgf. Both sides of the molded body were sanded with a sander to reduce the thickness to 12 mm, producing a molded plate.

Comparative Example 7-1 Production of molded plate by extrusion molding A molded plate was obtained in the same manner as in Example 7-1, except that the materials and contents shown in Table 6 below were used and calcium carbonate was not added.

<Evaluation of molded plates>
The molded plates produced by extrusion molding in the examples and comparative examples were evaluated as follows. The results are shown in Table 4.

(bulk density)
The bulk density was measured in accordance with JIS A 5430.

(Three-point bending test)
The three-point bending test was carried out in accordance with JIS A 5430. The results were taken as the strength (N/mm ² ) of the molded plate.

(heating test)
The heating test was conducted using the following equipment and procedure. Figure 8 is a partial perspective view showing a schematic diagram of the heating tester. As shown in Figure 8, an electric heater was used as the heat source, and refractory material was installed between the test specimen and the heat source to stabilize the temperature at around 900 °C, and the temperature of the backside of the test specimen was measured with a thermocouple. Specifically, an electric heater (1.2 kW heater) was used as the heat source equipment, and a K-type thermocouple and a temperature controller were connected. Each thermocouple was also connected to a data logger. The distance between the heating surface of the test specimen and the heat source was fixed at approximately 70 mm.

The test procedure was as follows.
(1) A scrap board was placed, preheating was performed up to 902°C, and then heating was performed once.
(2) The test piece was inserted after the heated surface had cooled to below 200°C.
(3) A thermocouple was placed in the center of the back surface (top surface in the figure) of the test specimen, and a calcium silicate plate (approximately 30 mm x 70 mm) and a weight were placed on top and fixed in place.
(4) Heating was started, and the temperature of the front and back sides was recorded with a data logger after the specified time (45 minutes). During this time, the temperature of the electric heater was set to 902°C on the heating surface side, and was controlled with a temperature controller with a lower limit of 900°C. The data logger also measured the temperature every 10 seconds, and the data was recorded at this interval.
(5) After the test was completed, the test specimen was removed and the following items were measured (each item was also measured before the test):
Dimensions: The length and width of the back surface and heated surface were measured with a vernier caliper. The area (mm ² ) of the heated surface before and after the test was calculated, and the heated surface shrinkage (%) was calculated based on the following formula.
Heating surface shrinkage (%) = {|S ₁ -S ₀ |/S ₀ } × 100
(Wherein, _S0 is the area of the heated surface before the test, and _S1 is the area of the heated surface after the test.)
Warpage: The test specimen was placed on an iron surface plate, and the height of the center of each side of the test specimen from the iron surface plate was measured with a thickness gauge, and the average value (mm) was calculated. This average value was defined as the warpage (mm) after heating.
- Photographs of the sample before and after the test (photos showing the extent of cracks before and after heating)

The molded plates of the Examples were superior to the Comparative Examples in all of strength, heated surface shrinkage, and post-heating warpage. In addition, the molded plates of the Examples did not develop cracks after heating (not shown).

Claims (8)

A method for producing calcium carbonate, comprising a step of contacting seawater, calcium-containing water remaining after producing magnesium hydroxide from seawater, or a mixture thereof with an alkaline agent and a gas containing carbon dioxide at a pH of 9 or higher and lower than 11,
the alkaline agent is magnesium hydroxide, magnesium oxide, or neutral magnesium carbonate ;
The method for producing calcium carbonate, wherein the calcium carbonate has an average particle size of 5 μm or less as determined by a laser diffraction method. 2. The method for producing calcium carbonate according to claim 1, wherein the seawater, utilized water, or a mixture thereof, the alkaline agent, and the gas are contacted in the following order 1) or 2):
1) After the seawater, utilized water or a mixture thereof is brought into contact with the alkaline agent, the gas is further brought into contact with the seawater, utilized water or a mixture thereof.
2) After the alkaline agent and the gas are brought into contact with each other, the seawater, utilized water, or a mixture thereof is further brought into contact with each other. The method for producing calcium carbonate according to claim 1 or 2, wherein the alkaline agent is magnesium hydroxide or magnesium oxide. The method for producing calcium carbonate according to claim 1 or 2, wherein the temperature in the contacting step is 50°C or less. The method for producing calcium carbonate according to claim 1 or 2, wherein the calcium concentration in the seawater, utilized water, or a mixture thereof is 300 ppm or more and 3,000 ppm or less. The method for producing calcium carbonate according to claim 1 or 2, wherein the gas is exhaust gas from a combustion engine. The method for producing calcium carbonate according to claim 1 or 2, wherein the concentration of carbon dioxide in the gas is 1% by volume or more and 20% by volume or less. The method for producing calcium carbonate according to claim 1 or 2, wherein the calcium carbonate is calcite.